Changes

Evolution influences every aspect of the form and behavior of organisms. Most prominent are the specific behavioral and physical [[adaptation]]s that are the outcome of natural selection. These adaptations increase fitness by aiding activities such as finding food, avoiding predators or attracting mates. Organisms can also respond to selection by [[Co-operation (evolution)|co-operating]] with each other, usually by aiding their relatives or engaging in mutually-beneficial [[symbiosis]]. In the longer term, evolution produces new species through splitting ancestral populations of organisms into new groups that are unable to breed with one another.

Evolution influences every aspect of the form and behavior of organisms. Most prominent are the specific behavioral and physical [[adaptation]]s that are the outcome of natural selection. These adaptations increase fitness by aiding activities such as finding food, avoiding predators or attracting mates. Organisms can also respond to selection by [[Co-operation (evolution)|co-operating]] with each other, usually by aiding their relatives or engaging in mutually-beneficial [[symbiosis]]. In the longer term, evolution produces new species through splitting ancestral populations of organisms into new groups that are unable to breed with one another.

These outcomes of evolution are sometimes divided into [[macroevolution]], which is evolution that occurs at or above the level of species, such as [[speciation]], and [[microevolution]], which is smaller evolutionary changes, such as adaptations, within a species or population. In general, macroevolution is the outcome of long periods of microevolution.<ref>{{cite journal |author=Hendry AP, Kinnison MT |title=An introduction to microevolution: rate, pattern, process |journal=Genetica |volume=112–113 |issue= |pages=1–8 |year=2001 |pmid=11838760}}</ref> Thus, the distinction between micro- and macroevolution is not a fundamental one - the difference is simply the time involved.<ref>{{cite journal |author=Leroi AM |title=The scale independence of evolution |journal=Evol. Dev. |volume=2 |issue=2 |pages=67–77 |year=2000 |pmid=11258392}}</ref> However, in macroevolution, the traits of the entire species are important. For instance, a large amount of variation among individuals allows a species to rapidly adapt to new habitats, lessening the chance of it going extinct, while a wide geographic range increases the chance of speciation, by making it more likely that part of the population will become isolated. In this sense, microevolution and macroevolution can sometimes be separate.<ref>{{wikiref |id=Gould-2002 |text=Gould 2002, pp. 657–658}}</ref>

These outcomes of evolution are sometimes divided into [[macroevolution]], which is evolution that occurs at or above the level of species, such as [[speciation]], and [[microevolution]], which is smaller evolutionary changes, such as adaptations, within a species or population. In general, macroevolution is the outcome of long periods of microevolution.(Hendry AP, Kinnison MT, An introduction to microevolution: rate, pattern, process |journal=Genetica |volume=112–113) Thus, the distinction between micro- and macroevolution is not a fundamental one - the difference is simply the time involved.<ref>{{cite journal |author=Leroi AM |title=The scale independence of evolution (Evol. Dev. |volume=2 |issue=2 |pages=67–77) However, in macroevolution, the traits of the entire species are important. For instance, a large amount of variation among individuals allows a species to rapidly adapt to new habitats, lessening the chance of it going extinct, while a wide geographic range increases the chance of speciation, by making it more likely that part of the population will become isolated. In this sense, microevolution and macroevolution can sometimes be separate.

A common misconception is that evolution is "progressive," but natural selection has no long-term goal and does not necessarily produce greater complexity.<ref>[http://www.sciam.com/askexpert_question.cfm?articleID=00071863-683B-1C72-9EB7809EC588F2D7 Scientific American; Biology: Is the human race evolving or devolving?], see also [[biological devolution]].</ref> Although [[evolution of complexity|complex species]] have evolved, this occurs as a side effect of the overall number of organisms increasing, and simple forms of life remain more common.<ref name=Carroll>{{cite journal |author=Carroll SB |title=Chance and necessity: the evolution of morphological complexity and diversity |journal=Nature |volume=409 |issue=6823 |pages=1102-09 |year=2001 |pmid=11234024}}</ref> For example, the overwhelming majority of species are microscopic [[prokaryote]]s, which form about half the world's biomass despite their small size,<ref>{{cite journal |author=Whitman W, Coleman D, Wiebe W |title=Prokaryotes: the unseen majority |url=http://www.pnas.org/cgi/content/full/95/12/6578 |journal=Proc Natl Acad Sci U S A |volume=95 |issue=12 |pages=6578–83 |year=1998|pmid=9618454}}</ref> and constitute the vast majority of Earth's biodiversity.<ref name=Schloss>{{cite journal |author=Schloss P, Handelsman J |title=Status of the microbial census |url=http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pubmed&pubmedid=15590780#r6 |journal=Microbiol Mol Biol Rev |volume=68 |issue=4 |pages=686–91 |year=2004 |pmid=15590780}}</ref> Simple organisms therefore remain the dominant form of life on Earth, and complex life appears more diverse only because it is [[biased sample|more noticeable]].<ref>{{cite journal |author=Nealson K |title=Post-Viking microbiology: new approaches, new data, new insights |journal=Orig Life Evol Biosph |volume=29 |issue=1 |pages=73–93 |year=1999 |pmid=11536899}}</ref>

A common misconception is that evolution is "progressive," but natural selection has no long-term goal and does not necessarily produce greater complexity.[http://www.sciam.com/askexpert_question.cfm?articleID=00071863-683B-1C72-9EB7809EC588F2D7 Scientific American; Biology: Is the human race evolving or devolving?], see also [[biological devolution]]. Although [[evolution of complexity|complex species]] have evolved, this occurs as a side effect of the overall number of organisms increasing, and simple forms of life remain more common. (Chance and necessity: the evolution of morphological complexity and diversity |journal=Nature |volume=409 |issue=6823 |pages=1102-09 |year=2001 |pmid=11234024) For example, the overwhelming majority of species are microscopic [[prokaryote]]s, which form about half the world's biomass despite their small size, (Whitman W, Coleman D, Wiebe W Prokaryotes: the unseen majority [http://www.pnas.org/cgi/content/full/95/12/6578] and constitute the vast majority of Earth's biodiversity.[http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pubmed&pubmedid=15590780#r6] Simple organisms therefore remain the dominant form of life on Earth, and complex life appears more diverse only because it is [[biased sample|more noticeable]].

===Adaptation===

===Adaptation===

Adaptations are structures or behaviors that enhance a specific function, causing organisms to become better at surviving and reproducing.<ref name=Darwin/> They are produced by a combination of the continuous production of small, random changes in traits, followed by natural selection of the variants best-suited for their environment.<ref>{{cite journal |author=Orr H |title=The genetic theory of adaptation: a brief history |journal=Nat. Rev. Genet. |volume=6 |issue=2 |pages=119–27 |year=2005 |pmid=15716908}}</ref> This process can cause either the gain of a new feature, or the loss of an ancestral feature. An example that shows both types of change is bacterial adaptation to [[antibiotic]] selection, with mutations causing [[antibiotic resistance]] by either modifying the target of the drug, or removing the transporters that allow the drug into the cell.<ref>{{cite journal |author=Nakajima A, Sugimoto Y, Yoneyama H, Nakae T |title=High-level fluoroquinolone resistance in Pseudomonas aeruginosa due to interplay of the MexAB-OprM efflux pump and the DNA gyrase mutation |url=http://www.jstage.jst.go.jp/article/mandi/46/6/46_391/_article/-char/en |journal=Microbiol. Immunol. |volume=46 |issue=6 |pages=391–95 |year=2002 |pmid=12153116}}</ref> However, many traits that appear to be simple adaptations are in fact [[exaptation]]s: structures originally adapted for one function, but which coincidentally became somewhat useful for some other function in the process.<ref name=GouldStructP1235>{{wikiref |id=Gould-2002 |text=Gould 2002, pp. 1235–1236}} </ref> One example is the African lizard ''Holapsis guentheri'', which developed an extremely flat head for hiding in crevices, as can be seen by looking at its near relatives. However, in this species, the head has become so flattened that it assists in gliding from tree to tree - an exaptation.<ref name=GouldStructP1235/>

Adaptations are structures or behaviors that enhance a specific function, causing organisms to become better at surviving and reproducing. They are produced by a combination of the continuous production of small, random changes in traits, followed by natural selection of the variants best-suited for their environment.(The genetic theory of adaptation: a brief history |journal=Nat. Rev. Genet. |volume=6 |issue=2 |pages=119–27 |year=2005 |pmid=15716908) This process can cause either the gain of a new feature, or the loss of an ancestral feature. An example that shows both types of change is bacterial adaptation to [[antibiotic]] selection, with mutations causing [[antibiotic resistance]] by either modifying the target of the drug, or removing the transporters that allow the drug into the cell.([http://www.jstage.jst.go.jp/article/mandi/46/6/46_391/_article/-char/en])  However, many traits that appear to be simple adaptations are in fact [[exaptation]]s: structures originally adapted for one function, but which coincidentally became somewhat useful for some other function in the process. One example is the African lizard ''Holapsis guentheri'', which developed an extremely flat head for hiding in crevices, as can be seen by looking at its near relatives. However, in this species, the head has become so flattened that it assists in gliding from tree to tree - an exaptation.

As adaptation occurs through the gradual modification of existing structures, structures with similar internal organization may have very different functions in related organisms. This is the result of a single [[homology (biology)|ancestral structure]] being adapted to function in different ways. The bones within bat wings, for example, are structurally similar to both human hands and seal flippers, due to the common descent of these structures from an ancestor that also had five digits at the end of each forelimb. Other idiosyncratic anatomical features, such as [[sesamoid bone|bones in the wrist]] of the [[panda]] being formed into a false "thumb," indicate that an organism's evolutionary lineage can limit what adaptations are possible.<ref>{{cite journal |author=Salesa MJ, Antón M, Peigné S, Morales J |title=Evidence of a false thumb in a fossil carnivore clarifies the evolution of pandas |url=http://www.pnas.org/cgi/content/full/103/2/379 |journal=[[Proceedings of the National Academy of Sciences|Proc. Natl. Acad. Sci. U.S.A.]] |volume=103 |issue=2 |pages=379–82 |year=2006 |pmid=16387860}}</ref>

As adaptation occurs through the gradual modification of existing structures, structures with similar internal organization may have very different functions in related organisms. This is the result of a single [[homology (biology)|ancestral structure]] being adapted to function in different ways. The bones within bat wings, for example, are structurally similar to both human hands and seal flippers, due to the common descent of these structures from an ancestor that also had five digits at the end of each forelimb. Other idiosyncratic anatomical features, such as [[sesamoid bone|bones in the wrist]] of the [[panda]] being formed into a false "thumb," indicate that an organism's evolutionary lineage can limit what adaptations are possible.<ref>{{cite journal |author=Salesa MJ, Antón M, Peigné S, Morales J |title=Evidence of a false thumb in a fossil carnivore clarifies the evolution of pandas |url=http://www.pnas.org/cgi/content/full/103/2/379 |journal=[[Proceedings of the National Academy of Sciences|Proc. Natl. Acad. Sci. U.S.A.]] |volume=103 |issue=2 |pages=379–82 |year=2006 |pmid=16387860}}</ref>